1. Technical Field
The present invention relates generally to integrated circuits, and more particularly to gate drive circuits utilized in integrated circuits for driving high-side N channel MOS power transistors. 2. Description of the Related Art
Power devices that have a high side N-channel MOS power transistor typically require a voltage level greater than the supply voltage to drive the gate of the N-channel MOS power transistor. One technique which has been used to derive the required increased voltage level is to employ a bootstrap capacitor. A disadvantage to using a bootstrap capacitor, however, is that it requires a large external capacitor. Furthermore, a circuit which utilizes a bootstrap capacitor can not be left on for long periods of time. As known in the art, the charge within the bootstrap capacitor drains off, and the bootstrap capacitor can not be recharged without turning the MOS power transistor off.
Another technique which is used to derive the required increase voltage level is a gate drive circuit, known in the art as a charge pump. A charge pump can be implemented either in a continuous running mode or as a circuit that works only on demand. Typically, a single charge pump is utilized in circuits which require charge pumps.
One disadvantage to using a single charge pump is that the switching time of the MOS power transistor is limited by the gate rise time produced by the charge pump. Additionally, when designing a charge pump, a tradeoff occurs between the current consumption of the charge pump versus the rise time of the gate of the MOS power transistor. This is especially true in NMOS designs where the pullup devices are resistor elements.
Therefore, it would be desirable to provide a charge pump circuit which reduces the gate rise time and the output switching time of a MOS power transistor. It is also desirable that the current consumption of the charge pump circuit be minimized.